The present invention relates to a frequency converter for an energy flow from a three-phase network to a consumer, for medium and high output voltages, with variable frequency, current and voltage outputs.
We know from the prior art that it is possible to drive three-phase medium voltage motors, i.e., motors for 3xc3x972.2 kV, 3xc3x973.3 kV, 3xc3x974.16 kV, 3xc3x976.3 kV, etc. up to 3xc3x9713.8 kV, by synthesizing the three-phase power using high-voltage semiconductors, IGBTs (insulated gate bipolar transistors), or IGCTs (integrated gate communicated transistors), or by cascading, i.e., series-connecting components intended for low voltages.
In those cases, system perturbations as well as motor voltages and motor currents must be taken into account. System perturbations have an effect on the line voltage and must therefore be avoided if possible. The quality of the line voltage is important. The total and individual harmonic factors are of interest for the line voltage. The total harmonic factor is the effective value of the ratio between the total harmonic and the total effective value. The individual harmonic factor is the effective value of the nth order harmonic in relation to the total effective value. The admissible maximum values of these quality parameters are defined in the relevant requirements. This means that the load currents of the medium voltage networks must be at least nearly sinusoidal.
Regarding the motor voltages, the differential quotient dv/dt must not be too high because high voltage and medium voltage motors are very sensitive to high dv/dt quotients, i.e., very fast voltage changes can lead to the destruction of the insulation of high and medium voltage motors. To avoid high dv/dt coefficients, filters are used between the motors and the associated DC/AC converters, for example when high-voltage components are used.
It is known in prior art to use asynchronous DC motors as medium voltage, variable speed drives for pumps, fans, etc. Such drives are not electrically braked, which means that kinetic energy is not fed back into the power grid. These are so-called two-quadrant drives.
DE 198 32 225 A1 discloses a four-quadrant converter for medium and higher voltages which is intended for single-phase or multi-phase consumers to generate outputs of variable amplitudes and frequencies. This converter consists of single-phase direct converter cells with a three-phase input from a transformer, with high-frequency filters and power switches. The single-phase direct converter cells are series-connected and fed via a star point of the corresponding secondary transformer winding. The high-frequency filters in the form of capacitors are delta or star connected with the associated secondary transformer windings. The power switches are bidirectional power semiconductor components or formed from equivalent circuits. In case of a three-phase design, the power outputs of the converter are star or delta connected.
DE 198 32 226 A1 discloses a four-quadrant converter for medium or higher voltages for single-phase and multi-phase consumers for generating outputs of variable amplitudes and frequencies. This converter is fed from a three-phase source of an isolation transformer and consists of a number of single-phase converter cells. The outputs of the single-phase converter cells are series-connected to generate high single-phase voltages and can be designed for the generation of direct current by star or delta connecting the corresponding outputs.
Thus, the two prior-art publications cited above describe high-voltage drives for asynchronous motors.
DE 196 35 606 A1 discloses an arrangement for generating a higher alternating current from several lower voltage direct-current sources. This prior-art arrangement is provided with one or more DC sources, a series connection, and voltage transformers which couple the DC sources to the series connection, which have power switches, and which generate partial voltages of variable width. The power switches are arranged in such a way that the partial voltages can be coupled to the series connection independently of each other. The series connection can be closed regardless of the number of coupled partial voltages. The voltage transformers consist of DC/AC converters generating partial AC voltages. This arrangement is particularly suitable for application in photovoltaic devices, where the DC/AC converters can be used as module-optimizing string converters.
U.S. Pat. No. 5,625,545 and U.S. Pat. No. 6,166,513 describe a two-quadrant AC/AC drive and a method for controlling AC motors. In these references, a multi-phase power transformer is provided with a large number of secondary windings for a large number of power cells. Each power cell has a single-phase output that is controllable via modulation control. The power cells are series-connected to facilitate a maximum output voltage in each cell. In this prior-art drive, the multi-phase power transformer, which is dimensioned for the full drive line, requires a corresponding number of secondary windings with complicated circuitry. Thus, some secondary windings are delta connected, others are star connected, while others are zigzag connected, etc.
Among known applications in the medium voltage range with asynchronous motors, are systems made by the firm of ABB under the name of ACS. For example, the company publication ABB Technik, No.6/1996, pages 31-29, describes AVS 600 drives with direct torque control. This and related systems use an input transformer which is designed for full motor power. These systems have 12-pulse diode rectifiers, intermediate-circuit capacitors, IGBT or IGCT converters and output filters. For example, the converter, which is formed by a voltage impressed converter, is series-connected with several high-voltage IGCTs. IGCTs for the medium voltage range have been described, for example, in the company publication ABB Technik, No. 3/1997, pages 12-17, under the title xe2x80x9cIGCTs megawatt semiconductor switch for the medium voltage rangexe2x80x9d.
High-voltage semiconductors, IGBTs and IGCTs have switching power losses that are higher by a factor of 3 to 10 than 1.2 kV and 1.7 kV IGBTs. Although their switching speed is not very fast, the voltage spike dv/dt is very large because the intermediate voltage is high, i.e., several kV. For an intermediate voltage of 100 kV and a switching time of 1 xcexcs, the dv/dt=100 kV/xcexcs. Such rapid voltage spikes cannot be tolerated by the windings of motors and/or transformers. To avoid such voltage spikes, additional passive LC filters, designed for fall power, are required at the output. All known solutions, as described above, use an input transformer. Some of these prior-art solutions, such as those of ABB, require output filters. U.S. Pat. No. 5,625,545 and U.S. Pat No. 6,166,513 propose an input transformer for full power, which has very complicated secondary windings, as mentioned above.
It is the object of the present invention to create a frequency converter for energy flow from a three-phase network to a consumer, for medium and high output voltages, with variable frequencies, currents and voltages, wherein the line currents with a symmetrical load of all network phases are at least nearly sinusoidal, whereby nearly sinusoidal load currents with variable outputs are generated, the voltage variation per time unit dv/dt at the output is relatively small to avoid insulation problems, and whereby commercially available highly-effective power semiconductor components can be used.
This object is achieved according to the invention by the characteristics of the invention.
The frequency converter according to the invention has the advantage that the line currents are at least nearly sinusoidal with a symmetrical load of all network phases, that at least nearly sinusoidal load currents with variable outputs of frequency, current and voltage are generated, that the voltage variation per time unit dv/dt at the output of the frequency converter is relatively small, so that insulation problems can be avoided, and that commercially available highly-effective power semiconductor components such as 1.2 kV IGBTs can be used.
The above, and other objects, features and advantages of the present invention, including details, characteristics and advantages will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.